1. Field of the Invention
The invention relates to mobile radio frequency supervision and, more particularly, mobile radio frequency drift supervision in a digital radio network.
2. History of the Prior Art
A fundamental concept underlying radio communications systems is that transmission and reception must occur at specific operating frequencies and that such frequencies must be stable over a period of time. Most systems achieve frequency stability by including a crystal oscillator which generates a very precise frequency which is then used to produce the various high frequency signals employed in the different components of the radio. Crystals are, however, subject to variations in their resonant frequency of oscillation due to various environmental conditions. Although various techniques are used to stabilize the frequency of oscillation of a crystal control oscillator, it may vary substantially and be unable to be compensated for due to failure of various components within the frequency compensation circuitry, poor design in the compensation circuitry, and for other reasons.
A mobile radio network may consist of a plurality of different systems, operated by different operators, each of which is made up of base stations and mobiles, many of which may be operating on the same relatively narrow frequency channels. In the event one of the mobile radios experiences transmitter frequency stability problems, it may begin transmitting on a different frequency from that which is intended and interfere with or totally block other transmissions on that channel. The location and identification of such offending mobile radio transmitters causing a disruption of communications within the radio network may be very difficult. Further, it is highly desirable to be able to detect such frequency instability in a mobile before communications in the overall network are disturbed or interrupted and summon the mobile station to a service center for adjustment of its transmitter frequency and/or promptly disable the transmitter of the mobile if it is already interfering substantially with the remainder of the communications traffic in the network.
In addition, there may be other reasons for seeking to disable a mobile from operation on the network. For example, in the event a subscriber fails to make timely payment of its bill for the mobile radio service, it may be desirable to disable the network from providing continued service to the mobile and force it to seek some explanation or correction of the disablement of the mobile at the network service center.
In digital packet radio communication systems, there are often a plurality of different systems working simultaneously on the same frequency channel within the radio network. Thus, the network must be careful to insure that the signal which is being transmitted by each mobile is within the frequency channels assigned to it by the network. Radio frequency interference and other spurious electrical signals will disrupt the communications channels to some extend and a mobile station with poor frequency stabilization makes broadcasting on the random channels even more difficult.
One resolution of this problem is to monitor the stability of the frequency of the signal broadcast by each of the mobiles in a network and disable and/or report a mobile who is sufficiently off frequency to cause communications difficulties in the network. When a base station attempts to measure the frequency being transmitted by a mobile and then disable the mobile in the event the frequency stabilization is not within the minimum accepted standards for the network, it must insure that only that particular mobile is disabled by the network. The base station must be careful not to mistake the signal of one mobile for another before it takes the drastic action of disabling the offending mobile.
Another aspect of digital packet radio systems which makes it difficult to use the signal transmitted from the mobile to measure the frequency and identify the mobile is that the data signals from the mobile are broadcast in burst mode. That is, the transmissions are all very short bursts of RF energy followed by the absence of RF signals in the spacing between bursts. This means that the base station must be capable of making a frequency measurement of the signal transmitted by the mobile very quickly. It must also measure the true mean frequency of the transmitter signal regardless of the digital information with which the carrier signal is modulated.
The system of the present invention overcomes these and other disadvantages of prior systems and enables the base station of a digital packet radio system to periodically measure the frequency of the signal being transmitted from each of the mobile stations and use that signal to determine whether or not the frequency standard being used by the mobile requires adjustment. In the event that the signal being broadcast is sufficiently erroneous that it may cause problems within the network, the base station may also notify the network control center to call the mobile into a service center for adjustment of its oscillator. Alternately, the network control center may disable the mobile from further transmitting operations. Monitoring and control of mobile frequency stability by the base stations of the network ensures that both the transmitting circuitry and the receiving circuitry of each mobile station is properly frequency stabilized against each of the potential variables which could cause the reference oscillator to be operating at less than a high degree of frequency stability and/or causing actual transmission difficulty within the network.